A wide variety of HUD systems are known. Commonly, a projection system is combined with a partial mirror (a partial reflector and partial window) as the final optical component for forming a projected image viewable by the user. Simultaneously, the user can view other scenes through the partial mirror. The partial mirror is an important component affecting the usability of the display. Generally, the reflectivity of the partial mirror must be sufficient to reflect light from the projector, but the partial mirror must also be sufficiently transparent to provide adequate viewing through it.
The partial mirror is typically placed on a windshield, canopy, or other transparent substrate, generically referred to herein as a window. Alternatively, the bare surface of the window is sometimes used as the partial mirror itself. In either case the combination of reflection from the partial mirror, and reflection from one or both of the inner and outer surfaces of the window, can produce a multiple image, or “ghosting” problem. This problem becomes more noticeable as the window thickness increases and the line-of-sight through the window becomes more oblique. For typical systems, ghosting problems become more noticeable for window thicknesses on the order of about 1 mm and up.
One known solution to this problem is to wedge the inner and outer surfaces of the window so that the (predominantly s-polarized) light reflecting off one surface is angularly separated from light reflecting off of the partial mirror or opposite surface. However, adding a wedge to a windshield can increase cost and manufacturing complexity. Further, achievable wedge angles for practical devices are very limited.
FIG. 1 depicts another known HUD system, disclosed in U.S. Pat. No. 5,999,314 (Asakura et al.). The system, S, includes a polarization-direction changing layer 2 bonded to the inboard surface of an outboard-side glass plate 1A which, together with an inboard-side glass plate 1B and an intermediate film 4 (polyvinyl butyral or the like), forms a laminated glass 5 used for an automotive front windshield glass W. Light from a displaying device 6 passes through a polarizer 7 so that the light is p-polarized (polarized in the plane of incidence). A Brewster's angle regulating film 3, made of TiO2, is formed at the inboard surface of plate 1B. P-polarized light from the displaying device 6 is incident on laminated glass 5 at Brewster's angle (θ=63°) of the Brewster's angle regulating film 3. No reflection is made at the inboard surface of plate 1B because the p-polarized light is incident at Brewster's angle. The light that thus enters laminated glass 5 then reaches polarization-direction changing layer 2, where the p-wave is rotated into an s-wave. The resulting s-polarized light is then partially reflected (about 20%) at an outboard surface of plate 1A. The reflected s-polarized light is re-converted to p-polarized light upon passing again through layer 2. This p-polarized light then passes through film 3 for observation by eyes 8 of the driver. The reference also discusses forming an optional reflection film 9 (a thin film of Al, Au, Ag, or Cu) at the outboard surface of plate 1A to further increase reflectivity. By eliminating reflection of light from displaying device 6 off the inboard surface of plate 1B, and by providing negligible reflection from other interfaces seen in the figure, the reference states that a double image cannot be formed.
One problem with the system of FIG. 1 is its reliance on reflection at an outer window surface, where water, ice, dirt, and the like can greatly distort or impair the image quality. Another problem is its reliance on a polarization-direction changing layer: such a layer that works well at large angles of incidence and over the entire visible spectrum is difficult to make, and is not generally available.
Other known HUD systems project other types of polarized light toward the viewer, such as s-polarized light or circularly polarized light. Generally, such systems suffer from the fact that some or all of the image light directed toward the viewer is polarized in a horizontal plane, which is the very polarization component rejected by ordinary polarized sunglasses. Thus if such sunglasses were used, the projected image would become substantially more difficult to see.
Thus, a need remains in the art for improved HUD systems in which ghosting is reduced or eliminated.